Concrete finishing trowel with speed control

ABSTRACT

A concrete finishing trowel includes a frame, a power source supported on the frame and having an output shaft, at least one rotor assembly supported on the frame. Each rotor assembly comprises a driven rotor shaft with a plurality of trowel blades extending outwardly therefrom so as to rest on a surface to be finished and to rotate with said driven rotor shaft to finish a respective circular area. A transmission system operably couples the power source output shaft to the driven rotor shaft of the at least one rotor assembly. The transmission system includes a variable driving pulley operable coupled directly or indirectly to the power source output shaft and including a resiliently deflectable portion that is movable to change the effective drive diameter of the variable driving pulley, and a variable driven pulley operably coupled to the variable driving pulley with a belt. The variable driven pulley in turn is operably coupled directly or indirectly to the driven rotor shaft of the at least one rotor assembly. A rotor speed control system permits an operator to adjust the center-to-center distance between the pulleys, thereby to cause the drive diameter of the variable driven pulley to change resulting in an adjustment of the speed of the driven rotor shaft.

FIELD OF THE INVENTION

The invention relates generally to concrete finishing trowels, and moreparticularly to a concrete finishing trowel having a rotor speed controlsystem and a concrete finishing trowel having a steering forceassistance system.

BACKGROUND OF THE INVENTION

Various machines are available for smoothing or otherwise finishing wetconcrete. Of the known varieties of such machines, self-propelledfinishing trowels, and particularly riding finishing trowels, areparticularly useful for finishing large sections of concrete morerapidly and efficiently than, for example, their manually-pushedcounterparts.

Riding concrete finishing trowels typically include a mobile frameincluding a deck, with two or more rotor assemblies mounted on anunderside of the deck. Each rotor assembly includes a driven rotor shaftextending down from the deck and a plurality of trowel blades mounted onand extending radially out from the bottom end of the driven rotorshaft. The trowel blades are supported on the surface to be finished.The driven rotor shafts of the rotor assemblies are driven by one ormore engines that also mounted on the frame of the finishing trowel. Theone or more engine is typically coupled to a respective driven rotorshaft via at least a gearbox. The weight of the finishing trowel and theoperator is transmitted frictionally to the concrete by the rotatingblades, thereby smoothing the concrete surface.

The individual blades usually can be tilted relative to their supports,via operation of a suitable mechanical lever and linkage systemaccessible by an operator seated on an operator's platform in order toalter the pitch of the blades. When this is done, the pressure appliedto the surface to be finished by the weight of the machine is altered.This blade pitch adjustment permits the finishing characteristics of themachine to be adjusted. For example, during a finishing operation, theoperator may first perform an initial “floating” operation in which theblades are operated at low speeds (on the order of about 30 rpm) but athigh torque. Then, the concrete is allowed to cure for another 15minutes to one-half hour, and the machine is operated at progressivelyincreasing speeds and progressively increasing blade pitches up to theperformance of a finishing or “burning” operation at the highestpossible speed. It is known to perform the burning operation atpreferably above about 150 rpm and up to about 200 rpm.

In order to provide for steering of a power finishing trowel, the drivenrotor shafts of the rotor assemblies are typically also tiltablerelative to the frame. By tilting the driven shafts of the rotorassemblies, the operator can cause the forces imposed on the concretesurface by the rotating blades to propel the vehicle in a directionextending perpendicularly to the direction of driven shaft tilt. Forexample, tilting at least the driven shaft of the rotor assembly fromside-to-side and fore-and-aft steers the vehicle in the forward/reverseand the left/right directions, respectively. Whereas the driven rotorshafts of both rotor assemblies should be tilted for forward/reversesteering control, it is known that only the driven shaft of one of therotor assemblies needs to be tilted for left/right steering control.

It is known to drive rotor assemblies of the typical riding finishingtrowel by a drive train that is connected directly to input shafts ofthe assemblies' gearboxes via a centrifugal clutch and a system ofshafts, belts or chains, and other torque transfer elements of constantspeed ratio. The drive trains typically require universal joints toaccommodate tilting of the gearbox relative to the remainder of thedrive train during a steering control operation. Alternatively, aflexible shaft may be employed, as discussed in U.S. Pat. No. 6,250,844to Sartler et al., the contents of which are incorporated entirelyherein by reference.

Various other proposals for drive trains in riding finishing trowelsinclude variable ratio transmissions, such as that disclosed in U.S.Pat. No. 5,967,696 to Allen et al. In the patent, Allen et al. discuss avariable gear drive unit comprising a variable ratio pulley driven by amotor. A second pulley drives the gear box input shaft, with a drivebelt entrained between the pulleys. A linear actuator causes the lineardisplacement of portions of the variable ratio pulley to change theeffective pulley diameter.

In known concrete finishing trowels, operators achieve variations inrotor speed by adjusting the speed of the engine. While indeed rotorspeed is adjustable in this way, there is the significant problem thatthe reduction in engine speed brings about a concomitant reduction indelivered power. This results in insufficient torque at the trowelblades. Therefore, improvements in the transmission of power from theengine to the driven rotor are desired.

Another issue known to those familiar with the concrete finishing trowelindustry is that, after having used riding finishing trowels for anextended period of time, an operator can become physically fatigued. Onecause of such physical fatigue is the requirement to repeatedly push andpull the steering handles against frictional forces so as to tilt thedriven rotors thereby to steer the machine. As such, improvements insteering control and comfort are therefore also desired.

It is therefore an object of the present invention to provide a novelconcrete finishing trowel.

SUMMARY OF THE INVENTION

Accordingly, in one aspect there is provided a concrete finishing trowelcomprising:

a frame;

a power source supported on the frame and having an output shaft;

at least one rotor assembly supported on the frame, each rotor assemblycomprising a driven rotor shaft with a plurality of trowel bladesextending outwardly therefrom so as to rest on a surface to be finishedand to rotate with said driven rotor shaft to finish a respectivecircular area;

a transmission system operably coupling the power source output shaft tothe driven rotor shaft of the at least one rotor assembly, comprising:

-   -   a variable driving pulley operable coupled directly or        indirectly to the power source output shaft and including a        resiliently deflectable portion that is movable to change the        effective drive diameter of the variable driving pulley;    -   a variable driven pulley operably coupled to the variable        driving pulley with a belt, and operably coupled directly or        indirectly to the driven rotor shaft of the at least one rotor        assembly; and

a rotor speed control system for permitting an operator to adjust thecenter-to-center distance between the pulleys,

wherein a change in the center-to-center distance causes the drivediameter of the variable driven pulley to change thereby to adjust thespeed of the driven rotor shaft.

The concrete finishing trowel described herein provides advantages overprior devices due at least in part to its ability to provide bothautomatic torque response and operator-adjustable speed control. Whereasprior concrete finishing trowels required the operator to achievevariable speeds by adjusting the speed of the engine itself, theconcrete finishing trowel described herein permits rotor speed controlwhile maintaining a level of engine speed that delivers sufficient powerand thus torque for the rotors to operate effectively during finishing.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described more fully with reference to theaccompanying drawings in which:

FIGS. 1 a to 1 d show various views of a concrete finishing trowelaccording to an aspect of the invention;

FIG. 2 is a perspective view isolating portions of the engine,transmission and rotor assembly portions of the concrete finishingtrowel of FIG. 1;

FIG. 3 is a perspective view isolating portions of the transmission androtor assemblies portions of the concrete finishing trowel of FIG. 1;

FIGS. 4 is a perspective view isolating portions of the steering systemand portions of the rotor assemblies of the concrete finishing trowel ofFIG. 1;

FIGS. 5 a and 5 b are a perspective views isolating portions of thesteering system including a steering assistance subsystem of theconcrete finishing trowel of FIG. 1; and

FIG. 6 is a perspective view isolating the steering assistance subsystemshown in FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning now to FIGS. 1 a to 1 d (hereinafter referred to as FIG. 1 forbrevity), a riding concrete finishing trowel in accordance with apreferred embodiment of the invention is shown. The riding concretefinishing trowel comprises a rigid frame, an upper deck mounted on theframe, an operator's platform or pedestal provided on the deck, andright and left rotor assemblies. The right and left rotor assembliesrespectively extend downwardly from the deck and support the finishingtrowel on the surface to be finished. The rotor assemblies rotatetowards the operator, or counterclockwise and clockwise, respectively,to perform a finishing operation.

A conventional ring guard is positioned at the outer perimeter of thefinishing trowel and extends downwardly from the deck to the vicinity ofthe surface to be finished. The pedestal is positioned longitudinallycentrally on the deck at a rear portion thereof and supports anoperator's seat. Preferably, the pedestal and operator's seat can bepivoted via hinges (not shown) to permit access to components of themachine located thereunder, such as the machine's engine. Vibrationdamping means are provided for reducing the vibration imparted to theoperator's seat. An operator presence switch is mounted beneath theoperator's seat. A fuel tank is disposed on the deck, beside thepedestal. On the opposite side of the pedestal is a fluid tank for anintegral retardant spray system. An electrical system, including fourheadlights for easing nighttime operation of the concrete finishingtrowel at the site, is protected against the unwanted ingress of waterand dust in accordance with the IP55 qualification set by InternationalElectrical Standards (IEC).

Each rotor assembly is coupled to a respective worm-drive gearbox. Thedriven rotor shaft of each rotor assembly extends downwardly from itsgearbox, and a plurality of circumferentially-spaced trowel blades aresupported on the driven rotor shaft via radial support arms. The trowelblades extend radially outwardly from the bottom end of the driven rotorshaft so as to rest on the concrete surface. As can be seen, in thisembodiment the trowel blades are mountable on multiple axial locationson the support arms, so as to enable an operator to alter the diameterof the circular area covered by each rotor assembly between finishingoperations. The circular areas may be made to overlap by selectiveplacement of the trowel blades on the support arms. Each gearbox ismounted on the undersurface of the deck but is tiltable relative to thedeck for steering, as will be described.

The pitch of the trowel blades of each of the right and left rotorassemblies can be individually adjusted by a dedicated blade pitchadjustment assembly. The pitch blade adjustment is preferablyelectrically actuated. Alternative blade pitch adjustment systems aredescribed in the above-noted U.S. Pat. No. 6,250,844 to Sartler et al.,and U.S. Pat. No. 2,887,934 to Whiteman, the contents of which areincorporated entirely herein by reference.

Both rotor assemblies, as well as other powered components of thefinishing trowel, are driven by a power source such as a gasolinepowered internal combustion engine that is mounted under the operator'sseat. It will be understood that the size of the engine will vary withthe size of the machine and the number of rotor assemblies powered bythe engine. For example, the illustrated two-rotor, 48″ machinetypically will employ an engine of about 25 hp. A catalytic convertersystem is coupled to the engine exhaust and supported on the frame.

The rotor assemblies are operably connected to the engine via a uniquetransmission system and, along with their respective gearboxes, can betilted for steering purposes via a unique steering system as will bedescribed.

FIG. 2 is a perspective view isolating portions of the engine mounted onthe frame, the transmission and the rotor assembly portions of theconcrete finishing trowel of FIG. 1. FIG. 3 shows the various isolatedportions without the engine. The output shaft of the engine is coupledto a clutch, which in turn is coupled to a driven pulley via a belt. Thedriven pulley is rotatably mounted on a jack shaft that is supportedwithin the subframe of the concrete finishing trowel. The other end ofthe jack shaft is connected to a variable driving pulley which, via avariable driving belt, is coupled to a variable driven pulley. A U-jointdrive shaft is coupled to the variable driven pulley via a shear pinhub, and extends to the tiltable right rotor assembly via its respectivetiltable worm gearbox. The shear pin hub receives a shear pin which actsas a mechanical fuse of sorts to mechanically couple the rotorassemblies to the power source. As would be understood, the shear pin isdesigned to break under predetermined conditions such as overload orimpact, thereby to impart little or no damage through the transmissionsystem to the engine. A broken shear pin is easily replaced by anoperator in the field and thus is a simple, inexpensive and effectiveway by which certain components of the concrete finishing trowel can beshielded from costly damage.

Another U-joint drive shaft extends from the variable driven pulley toanother tiltable worm gearbox which is, in turn, coupled to the leftdriven rotor. During a steering operation, the tiltable worm gearboxesmove in unison with their respective rotor assemblies, as permitted bythe U-joint drive shaft.

In operation, rotational power is transmitted via the driven pulley tothe variable driving pulley, then to the variable driven pulley via thevariable driving belt, via the U-joint drive shafts to respective wormgearbox and finally to the respective rotor driven shafts of the rotorassemblies.

The variable driving pulley includes a resiliently deflectable portionthat is automatically movable to change the effective drive diameter ofthe variable driving pulley in response to changes in torque. In thisembodiment, the resiliently deflectable portion is one half of thepulley that is biased to a “home” position relative to the other half ofthe pulley. When the deflectable pulley half moves away from its otherhalf, the variable driven pulley belt traverses a smaller pulleydiameter, thus causing the variable driven pulley, and accordingly thedriven rotor, to turn slower. A spring biases the deflectable pulleyhalf back towards its home position. As the deflectable pulley halfmoves back towards the home position, the variable driven pulley belttraverses a larger pulley diameter, thus causing the variable drivenpulley and accordingly the driven rotor, to turn faster.

The resiliently deflectable portion will move automatically as theoperating conditions fluctuate, accordingly widening the space betweenthe pulley halves. Thus, such changes permit the engine speed to remainconstant through changes in the rotational speed of the rotors.

Advantageously, the concrete finishing trowel is provided with a rotorspeed control system that permits an operator of the machine to adjustthe speed of the driven rotors without necessarily adjusting enginespeed. In order to decrease the driven rotor speed of rotation, theoperator turns a speed control handle (shown as item 1 in FIG. 3)clockwise. Turning the speed control handle clockwise causes a yoke endto move upwards. This counterclockwise turns a front control shaft towhich the yoke end is connected (when viewed from the right of themachine in FIG. 3). Turning of this front control shaft causes the tierod to which it is connected to shift to the left (again, when viewedfrom the right of the machine) causing in turn the rear control shaft towhich the tie rod is connected to rotate counterclockwise. As the rearcontrol shaft is rotated counterclockwise, the shaft supporter thatsupports the shaft connecting the driven pulley and the variable drivingpulley is moved upwards. As the shaft is moved upwards, the variabledriving pulley is accordingly caused to move away from the variabledriven pulley. More particularly, the center to center distance betweenthe variable driving pulley and the variable driven pulley is increased.At this point, the tension on the variable driving belt increases so asto cause it to force itself between the two pulley halves of thevariable driving pulley. As a result, the deflectable pulley half ispushed away from its other half, and the effective pulley diameter isdecreased. This decrease leads to a decrease in the speed of thevariable driving belt and accordingly a decrease in the speed of thedriven rotors.

In order to increase the driven rotor speed of rotation, the operatorturns the speed control handle (shown as item 1 in FIG. 3)counterclockwise. Turning the speed control handle counterclockwisecauses the yoke end to move downwards. This clockwise turns the frontcontrol shaft to which the yoke end is connected (when viewed from theright of the machine in FIG. 3). Turning of this front control shaftcauses the tie rod to which it is connected to shift to the right(again, when viewed from the right of the machine) causing in turn therear control shaft to which the tie rod is connected to rotateclockwise. As the rear control shaft is rotated clockwise, the shaftsupporter that supports the shaft connecting the driven pulley and thevariable driving pulley is moved downwards. As the shaft is moveddownwards, the variable driving pulley is accordingly caused to movetowards the variable driven pulley. More particularly, the center tocenter distance between the variable driving pulley and the variabledriven pulley is decreased. At this point, the tension on the variabledriving belt decreases and due to the spring-influenced resilience ofthe deflectable pulley half the pulley halves are moved closer together.As a result, the effective pulley diameter is increased. This increaseleads to an increase in the speed of the variable driving belt andaccordingly an increase in the speed of the driven rotors.

It will be understood that, at any given speed set by the operator ofthe concrete finishing trowel, the transmission system will still haveautomatic torque response because the deflectable pulley half will beable to move relative to its other half in response to changingconditions. As such, a manually adjustable speed control with integratedautomatic torque response has been provided in a triple reductiontransmission system

FIGS. 4 is a perspective view isolating portions of the steering systemand portions of the rotor assemblies of the concrete finishing trowel ofFIG. 1. The steering system comprises a pitch sensor bracket, a limitswitch adjustment bracket, a gearmotor, a pin-block U-joint, a screwjack, a jack mount bracket, a yoke arm, a limit switch, a pressureplate, a switch mount bracket, and a steering control arm. The steeringsystem is supported on the subframe of the concrete finishing trowel.

FIGS. 5 a and 5 b are perspective views isolating portions of thesteering system including a steering assistance subsystem of theconcrete finishing trowel. A control panel supports operator hand gripsand gauges for giving an operator visual indications of trowel bladespeed in revolutions per minute, the relative degree of blade pitch, thebattery charging status, the fuel level, the total run hours, a “systemon” indicator lamp, and a “low oil pressure” indicator lamp. It will beunderstood that other indicators and controls can be supported on thecontrol panel. The control panel is supported atop a single steeringcolumn, which is pivotable in various directions to, in conjunction witha steering assistance system (shown in isolation in FIG. 6) change theposition of left- and right-side steering control arms in order toaccordingly tilt the driven rotors in the rotor assemblies to effectsteering. The left-side steering control arm is mounted onto the back ofthe left side gearbox, while the right-side steering control arm ismounted on the back of the right side gearbox. A steering bracket isfixed to the vertical steering shaft within the steering column, andthere is no relative movement between the steering bracket and thesteering shaft.

In order to propel the concrete finishing trowel forward duringoperation, the operator grips both hand grips and pushes the steeringcolumn forward so as to pivot within a spherical bearing (see item 13 inFIG. 6). The steering bracket 14 then moves forward and the ends of therods 26 and 27 move forward. In turn, the steering shafts 9 and 28 areturned counterclockwise (when viewing FIG. 5 a from the left side), andthe pivot plates 3 and 29 are turned clockwise. Ends of rods 2 and 30then move downwards and both gearboxes are tilted so as to cause theconcrete finishing trowel forwards.

In order to propel the concrete finishing trowel backwards duringoperation, the operator grips both hand grips and pulls the steeringcolumn backward so as to pivot within a spherical bearing (see item 13in FIG. 6). The steering bracket 14 then moves backwards and the ends ofthe rods 26 and 27 move backwards. In turn, the steering shafts 9 and 28are turned clockwise (when viewing FIG. 5 a from the left side), and thepivot plates 3 and 29 are turned counterclockwise. Ends of rods 2 and 30then move upwards and both gearboxes are tilted so as to cause theconcrete finishing trowel to travel backwards.

In order to turn the concrete finishing trowel to the left, the operatorpulls the left hand grip and pushes the right hand grip. The verticalsteering shaft accordingly is turned counterclockwise (when viewed fromthe top). The rod end 26 then moves backwards and rod end 27 movesforwards. Accordingly, the left side steering shaft 9 turns clockwisewhile the right side steering shaft 28 turns counterclockwise. When thisoccurs, the left side pivot plate 3 turns counterclockwise and the rightside pivot plate turns clockwise. As this occurs, the rod end 2 moves upand the rod end 30 moves down. The left side gearbox accordingly tiltsoutside and the right side gearbox tilts inside, causing the concretefinishing trowel to turn left.

In order to turn the concrete finishing trowel to the right, theoperator pushes the left hand grip and pulls the right hand grip. Thevertical steering shaft accordingly is turned clockwise (when viewedfrom the top). The rod end 26 then moves forwards and rod end 27 movesbackwards. Accordingly, the left side steering shaft 9 turnscounterclockwise while the right side steering shaft 28 turns clockwise.When this occurs, the left side pivot plate 3 turns clockwise and theright side pivot plate turns counterclockwise. As this occurs, the rodend 2 moves down and the rod end 30 moves up. The left side gearboxaccordingly tilts inside and the right side gearbox tilts outside,causing the concrete finishing trowel to turn right.

In order to move the concrete finishing trowel laterally to the left,the operator moves both hand grips to the left. The vertical steeringshaft is thereby caused to move leftwards about its pivot point on thespherical bearing 13, and the steering bracket 14 accordingly movesleft. The rod end 24 moves left, while the L/R pivot plate 17 turnscounterclockwise. The L/R steering lever 8 then turns counterclockwiseand the rod end 25 moves up. Accordingly, the right side gearbox iscaused to tilt backwards (when viewed from the top), causing theconcrete finishing trowel to move left laterally.

In order to move the concrete finishing trowel laterally to the right,the operator moves both hand grips to the right. The vertical steeringshaft is thereby caused to move rightwards about its pivot point on thespherical bearing 13, and the steering bracket 14 accordingly movesright. The rod end 24 moves right, while the L/R pivot plate 17 turnsclockwise. The L/R steering lever 8 then turns clockwise and the rod end25 moves down. Accordingly, the right side gearbox is caused to tiltforwards (when viewed from the top), causing the concrete finishingtrowel to move right laterally.

FIG. 6 is a perspective view isolating the steering assistance subsystemshown in FIGS. 5 a and 5 b. The steering assistance subsystem makes iteasier for an operator to move the vertical steering column from itsneutral position. Springs 16 are compressed, such compression beingadjustable by way of movable adjusting nuts 18 and lock nuts 19 beingmoved up- or downwards. A cross pivot bracket 22 is pivotable about pin32 on vertical steering shaft 12, and pivot bracket 15 is pivotableabout the pins on cross pivot bracket 22. As a result, pivot bracket 15can remain parallel to the pressure plat 10 under the equal pressure ofthe four springs 16, while the vertical steering shaft 12 moves in thespherical bearing 13.

At a neutral position, the four springs are compressed, and accordinglypush the pivot bracket vertically downwards to the centre of thespherical bearing 13. When the vertical steering shaft is moved eitherbackwards or forwards by the operator of the concrete finishing trowel,the pivot bracket moves forwards or backwards causing the springs toleave their neutral position. The springs extend and provide a level ofassisting force to push the steering shaft in the direction in which ithas been moved. As a result, the vertical steering shaft tends to remainin the position in which it is moved by the operator to provideassistance to the operator. A similar operation occurs when the verticalsteering shaft is moved either backwards or forwards.

When the vertical steering shaft is twisted as described above, thepivot bracket is turned, causing the springs to extend and providetwisting force assistance.

While the above has been set out with reference to an embodiment, itwill be understood that alternative embodiments that fall within thepurpose of the invention set forth herein are possible.

Although embodiments have been described with reference to the drawings,those of skill in the art will appreciate that variations andmodifications may be made without departing from the spirit and scopethereof as defined by the appended claims.

1. A concrete finishing trowel comprising: a frame; a power sourcesupported on the frame and having an output shaft; at least one rotorassembly supported on the frame, each rotor assembly comprising a drivenrotor shaft with a plurality of trowel blades extending outwardlytherefrom so as to rest on a surface to be finished and to rotate withsaid driven rotor shaft to finish a respective circular area; atransmission system operably coupling the power source output shaft tothe driven rotor shaft of the at least one rotor assembly, comprising: avariable driving pulley operable coupled directly or indirectly to thepower source output shaft and including a resiliently deflectableportion that is movable to change the effective drive diameter of thevariable driving pulley; and a variable driven pulley operably coupledto the variable driving pulley with a belt, and operably coupleddirectly or indirectly to the driven rotor shaft of the at least onerotor assembly; and a rotor speed control system for permitting anoperator to adjust the center-to-center distance between the pulleys,wherein a change in the center-to-center distance causes the drivediameter of the variable driven pulley to change thereby to adjust thespeed of the driven rotor shaft.
 2. The concrete finishing trowel asdefined in claim 1, wherein said power source comprises an internalcombustion engine.
 3. A concrete finishing trowel as defined in claim 1,wherein said transmission system further comprises a gearbox from whichsaid driven rotor shaft extends and which tilts relative to said frameduring a steering operation, said gearbox having an input shaft which isoperatively coupled to said variable driven pulley via a U-joint driveshaft.
 4. A concrete finishing trowel as defined in claim 1, whereineach of the at least one rotor assembly further comprises a plurality ofsupport arms which extend radially outwardly from said driven shaft andon which said trowel blades are mounted, and wherein said trowel bladesare mountable on multiple axial locations on said support arms so as toalter the diameter of said circular area.
 5. A concrete finishing trowelas defined in claim 4, wherein said finishing trowel is a riding trowelcomprising at least two rotor assemblies each for finishing a respectivecircular area.
 6. A finishing trowel as defined in claim 5 and furthercomprising: a deck mounted on the frame; an operator's pedestalpositioned on said deck; and an operator's seat supported by saidpedestal; wherein said pedestal and said seat are hingedly attached tosaid deck to permit access to components of said finishing trowellocated thereunder.
 7. The concrete finishing trowel of claim 1, furthercomprising: a shear pin hub associated with the variable driven pulley,the shear pin hub adapted to receive a shear pin for mechanicallycoupling the at least one rotor assembly to the power source, wherein ashear pin is designed to break under predetermined conditions thereby tomechanically decouple the at least one rotor assembly from the powersource with little damage.
 8. A concrete finishing trowel comprising: aframe; a power source supported on the frame and having an output shaft;at least one rotor assembly supported on the frame, each rotor assemblycomprising a driven rotor shaft with a plurality of trowel bladesextending outwardly therefrom so as to rest on a surface to be finishedand to rotate with said driven rotor shaft to finish a respectivecircular area; a transmission system operably coupling the power sourceoutput shaft to the driven rotor shaft of the at least one rotorassembly; a steering system comprising a steering column operablycoupled to the rotor assembly and operable to tilt the rotor drivenshaft relative to the frame thereby to effect a steering operation, thesteering system comprising a steering assistance subsystem operable toprovide mechanical assistance to aid an operator when moving thesteering column away from a neutral position.
 9. The concrete finishingtrowel of claim 8, wherein the transmission system comprises: a variabledriving pulley operable coupled directly or indirectly to the powersource output shaft and including a resiliently deflectable portion thatis movable to change the effective drive diameter of the variabledriving pulley; a variable driven pulley operably coupled to thevariable driving pulley with a belt, and operably coupled directly orindirectly to the driven rotor shaft of the at least one rotor assembly;the concrete finishing trowel further comprising: a rotor speed controlsystem for permitting an operator to adjust the center-to-centerdistance between the pulleys, wherein a change in the center-to-centerdistance causes the drive diameter of the variable driven pulley tochange thereby to adjust the speed of the driven rotor shaft.